This invention relates generally to vision systems and more particularly to a method and system for gating a power supply in a radiation detector.
There are numerous methods and systems for detecting radiation. In one such system, an image intensifier tube is used to amplify light and allow a user to see images in very dark conditions. These night vision devices typically include a lens to focus light onto the light receiving end of an image intensifier tube and an eyepiece at the other end to view the enhanced imaged produced by the image intensifier tube.
Current image intensifier tubes use photocathodes. Photocathodes emit electrons in response to photons impinging on the photocathodes. The electrons are produced in a pattern that replicates the original scene. The electrons from the photocathode are accelerated towards a microchannel plate. A microchannel plate is typically manufactured from lead glass and has a multitude of microchannels, each one operable to produce a cascade of secondary electrons in response to an incident electron. Therefore, photons impinge on the photocathode producing electrons which are then accelerated to a microchannel plate where a cascade of secondary electrons are produced. These electrons impinge on a phosphorous screen, producing an image of the scene.
Current image intensifier tubes also provide automatic brightness control (ABC) and bright source protection (BSP). ABC maintains a relatively constant level of brightness in the image produced by the image intensifier tube despite fluctuating levels of brightness in the scene being viewed. BSP prevents the image intensifier tube from being damaged by high levels of current that may otherwise be generated in response to an extremely bright source.
Currently available image intensifier tubes provide ABC and BSP by gating a power supply to the photocathode. The term xe2x80x9cgatingxe2x80x9d as used herein refers to the enabling or disabling of the photocathode of an image intensifier by providing an on-state voltage or an off-state voltage to the photocathode with respect to the input of the microchannel plate. These currently available image intensifier tubes generally utilize optical or magnetic coupling for activation of the gating circuit, which is typically floating with respect to the input of the microchannel plate. Drawbacks to these approaches include relatively slow rise/fall times and long delay times, as well as relatively great power requirements and a large number of components. Optical coupling also requires floating low voltage supplies for biasing.
In accordance with the present invention, the disadvantages and problems associated with previous image intensifiers have been substantially reduced or eliminated. In particular, the present invention provides an improved method and system for gating a power supply in a radiation detector such as an image intensifier.
In one embodiment, a method is provided for gating a power supply. A gating command terminal is capacitively coupled to an on/off switch. A gating command signal is provided at the gating command terminal. The gating command signal is operable to activate the on/off switch. An off signal is generated at an output terminal of the on/off switch in response to an off state of the on/off switch being activated. An on signal is generated at the output terminal in response to an on state of the on/off switch being activated.
Technical advantages of the present invention include providing an image intensifier with improved automatic brightness control and bright source protection. In particular, an image intensifier provides automatic brightness control and bright source protection by gating a power supply to a photocathode utilizing capacitive coupling. As a result, the design is highly compact, drop-in replacement is possible, rise/fall times are decreased, and delay times are reduced.
Other technical advantages include the use of momentary switch action that is possible with transistors. As a result, high speed switching is provided, while a high output impedance for the photocathode is maintained to provide flash protection.
Other technical advantages of the present invention will be readily apparent to those skilled in the art from the following figures, descriptions and claims.